The present invention relates to a new and improved construction of sample and hold-circuit arrangement for an electrical motor simulator in an electronic motor protection or protective relay wherein the motor simulator is charged with a clocked charging current by a charging current source through a main or primary switch controlled by clock signals.
In an electronic motor protection relay the charging voltage of an electrical motor simulator composed of capacitors and resistors is compared with a comparison voltage characteristic of the permissible motor threshold or boundary temperature in order to obtain a control signal for cutting-off the motor current in the presence of impermissibly high heating-up of the motor. The electrical simulation of the thermal behavior of a three-phase motor requires large time-constants which cannot be advantageously realized by utilizing extremely high-ohm resistors and very weak charging currents, but rather by supplying the motor simulator with clocked charging current. If there is taken into account the different thermal behavior of a running motor and a stationary motor or of a motor which is to be considered as stationary, then as far as the motor simulator is concerned there also must be provided appropriately different time-constants. To that end there must be present in the motor simulator switching means which can be turned-on and turned-off for increasing the virtual impedance of the resistance network of the motor simulator, as such has been described in detail for instance in Swiss Pat. No. 540,587, the disclosure of which is incorporated herein by reference. Thus, for instance, the time-constant of the motor simulator is increased in that its resistance network is connected by means of its low or bottom end at a fraction of the charging voltage which can be tapped-off by means of an impedance converter and a voltage divider. In both cases, that is to say, for the derivation of a cut-off control signal and for influencing the time-constants of the motor simulator there is accordingly required in the electronic motor protection relay a circuit arrangement which in an appropriate manner makes available a voltage which is equal to, or proportional to the charging voltage of the motor simulator.